GB812963A - Manufacture of low molecular carboxylic acid anhydrides - Google Patents

Abstract

Lower carboxylic acid anhydrides are prepared by thermally dehydrating a saturated aliphatic monocarboxylic acid containing 2-4 carbon atoms and being initially substantially free of the corresponding anhydride, by heating the acid in the boiling liquid phase at a temperature of at least 180 DEG C. and under an elevated pressure of about 2 to about 50 atmos. gauge, the temperature and pressure used being such that the anhydride formed remains behind in the liquid phase, and distilling off the water formed in admixture with acid from the reaction mixture through a distillation column during the dehydration, at least part of the distilled acid being returned to the distillation column. Advantageously, a water-entraining agent may be added to the carboxylic acid, a mixture of entraining agent, water and acid then being distilled over. The entrainer may be an aliphatic, cycloaliphatic or aromatic hydrocarbon or an ester, such as cyclohexane, n-hexane, benzene and various methyl, ethyl, propyl and butyl esters of acetic, propionic and butyric acids. The entrainer is preferably one which is immiscible with water, forms an azeotrope therewith and boils at least 20 DEG C. below the B.P. of the acid. Preferably, the condensate of the azeotropic mixture should separate into an aqueous phase and a non-aqueous phase, the latter comprising entrainer and acid being returned to the distillation column wholly or in part. When the mixture distilled over is homogeneous, for example when an entraining agent is not used, the distillate can be freed from water in a second distillation under normal pressure and the recovered anhydrous acid pumped to the main distillation column; alternatively part only of the homogeneous distillate may be recycled and the remainder withdrawn from the system. Suitable apparatus for discontinuous distillation comprises a still having a packed column, a condenser and a separator for use when a two-phase condensate is obtained, the apparatus being connected to a pressure vessel. The walls of the apparatus are preferably made of material having substantially no catalytic effect on decomposition of the organic materials, e.g. copper, enamelled iron, silver, tantalum or Cr- Ni- Mo-steel. Dehydration catalysts may be present. With discontinuous distillation, formation of anhydride practically ceases when a certain degree of conversion has been attained. The process, however, may be made continuous by running-off the liquid anhydride-containing product from the still, separating the unchanged acid from anhydride in another column at atmospheric pressure and returning the acid to the pressure column. Anhydrides and mixed anhydrides may be prepared from acetic, propionic and n- or isobutyric acids. In typical examples: (1) a mixture of acetic acid and benzene is boiled at 210 DEG C. under a nitrogen pressure of 10 atmos. in the above described apparatus; a mixture of benzene, water and acetic acid distills off, is condensed and an aqueous acid phase is separated from an organic phase of benzene and acetic acid which is returned to the column; (2) a mixture of acetic acid and cyclohexane is distilled in the same apparatus at 250 DEG C. under 17 atmos. inert gas pressure and part of the homogeneous condensate is withdrawn and the remainder recycled to the column; (3) acetic acid is distilled in the same apparatus at 230 DEG C. under 10 atmos. inert gas pressure in the absence of an entrainer, part of the distillate being withdrawn and the remainder recycled; (4) following the procedure of (3) propionic acid is converted into its anhydride at 235 DEG C. under 8 atmos. pressure and butyric acid into its anhydride at 230 DEG C. under 3.5 atmos. pressure.